1. Technical Field of the Invention
The present invention relates to rotary electric machines and, more particularly, to a rotary electric machine, having a commutator, and a commutator for such a rotary electric machine.
2. Description of the Related Art
In recent years, for rotary electric machines (in particular, starters) installed on vehicles, there have been needs for the rotary electric machines to have structures each in a small size and lightweight with an aim of limiting an installation space and improving fuel consumption. As one example of expedients for addressing such needs, an attempt has heretofore been made to provide a rotary electric machine that includes armature coils each adopting a rectangular wire to have an increased occupation factor while providing increased output torque.
Meanwhile, when making wire connection between the armature coils and a commutator, it has been a general practice for the commutator to have a plurality of commutator segments each formed with a riser portion. The riser portion is formed with a slot to which end portions of the armature coils are inserted and connected to the riser portion by fusing. With the armature coils employing the rectangular wires, the rectangular wires are inserted to the slot of the riser portion such that longitudinal axes of the rectangular wires in cross section are placed in the slot along a radial direction. With such a structure, the riser portions of the commutator segments necessarily have larger size in outer diameter than those of the armature coils each employing a rounded wire. This results in an increase in a mass of the riser portions, making it difficult to make a design with centrifugal force resistance.
As one of expedients for the commutator to endure a centrifugal force, an attempt has been made to provide a commutator of the type disclosed in U.S. Pat. No. 6,744,169. The commutator of such a related art includes plural commutator segments. Each commutator segment has radially inward ends formed with axially extending projecting portions. The axially extending projecting portions are embedded in an insulating mold resin. Moreover, each commutator segment has an anchor portion, embedded in the mold resin, which has both axial ends formed with substantially C-shaped depressed portions on which circular metallic bands are disposed. This allows the commutator segments to be firmly retained in the mold resin to endure a high centrifugal force.
With the commutator of the related art mentioned above, the use of metallic bands results in an increase in the number of component parts. Also, this results in an increase in a production man-hour. These cause an issue to arise with the occurrence of an increase in cost.
To address such an issue, another attempt has heretofore been made to provide a commutator that includes plural commutator segments. Each commutator segment has a radially inward portion, embedded in a mold resin, which is formed with a tab tail to endure a high centrifugal force. The commutator segment has no need to use separate component elements such as metallic bands or the like, causing no increase in the number of component parts and production man-hour while enabling the production of the commutator at low cost. As set forth above, however, in a case where the armature coils adopt the rectangular wires, the riser portions of the commutator segments increase in outer diameter with an increase in mass. This causes an increase in centrifugal force acting on the mold resin via the riser portions. This results in the occurrence of concentrated stress acting on the mold resin in areas retaining radially inward areas of the riser portions. This causes a cracking to occur in the mold resin, resulting in an issue to arise with a difficulty of enduring a centrifugal force.